1. Technical Field of the Invention
The present invention relates to circuit and method for writing to a memory disk, and particularly to a circuit and method for differentially driving the write head of a disk drive device.
2. Background of the Invention
Most computer systems include one or more associated disk drives, which may be built into or external to the computer system. Typically, disk drives have at least one rotating magnetic medium and associated head mechanisms that are carried adjacent the magnetic material. The heads are radially positionable to selectively write information to, or read information from, precise positions on the disk medium. Such disk drives may be, for example, hard disk drives, floppy drives, or the like.
Data is written to the associated data disk by applying a series of signals to a write head according to the digital information to be stored on the magnetic disk media. The write head has a coil and one or more associated pole pieces that are located in close proximity to the disk media. As signals cause the magnetic flux to change in the head, the magnetic domains of the magnetic media of the disk are aligned in predetermined directions for subsequent read operations. Typically, a small space of unaligned magnetic media separates each magnetic domain transition to enable successive transitions on the magnetic media to be distinguished from each other.
Since the disk is moving relative to the head, it can be seen that if the small space separating the magnetic domain transitions is not sufficiently wide, difficulty may be encountered in distinguishing successive magnetic transitions. This may result in errors in reading the data contained on the disk, which is, of course, undesirable.
Meanwhile, as computers are becoming faster, it is becoming increasingly important to increase the speed at which data can be written to and read from the disk media. However, since the data signals are in the form of square wave transitions, if the rise time of the leading edges of the square waves is large, the small space between magnetic media transitions also becomes large, which reduces the effective rate at which data can be accurately written and read. Since the write head assembly includes at least one coil, forcing the current to rise rapidly, or to reverse flux directions within the write head is difficult.
In the past, data writing circuits and/or write drive circuits used to supply such write signals to the heads included preamplifier circuits to drive the current through selected legs of an “H-bridge” circuit, which is capable of allowing relatively fast current reversals for accurate data reproduction.
An example of a typical H-bridge write head drive circuit 10, according to the prior art, is shown in FIG. 1. The circuit 10 includes four MOS transistors, 12-15 connected between a high reference voltage Vcc and a low reference voltage Vss. A coil 19, used, for example, to supply data pulses for writing to a disk drive media is integrated into the write head mechanism. The coil 19 is connected between the center legs of the H-bridge, as shown.
It can been seen that, depending on the gate biases applied to the respective transistors 12-15, the current flows through the coil 19 in one direction or another. That is, one current flow path includes the transistor 14, coil 19 from right to left, and transistor 13. The other current flow path includes transistor 12, the coil 19 from left to right, and the transistor 15.
In the H-bridge circuit 10, the transistor 12 and 14 serve as switching transistors, which are controlled by the out-of-phase signals on a pair of respective input lines 28 and 29. The transistors 13 and 15 serve as current controlling transistors, which are controlled by the out-of-phase signals on the respective input lines 29 and 28 in a manner opposite from the connections to the switching transistors 12 and 14, via respective control transistors 31 and 32. The magnitude of the current through the transistors 13 and 15 is controlled by a transistor 21, with which the transistors 13 and 15 form respective current mirrors, when connected via respective transmission gates 24 and 25. The transmission gates 24 and 25 are controlled by the signals on the respective input lines 29 and 28, in the same manner as the associated transistors 31 and 32. A reference current source 26 supplies the reference current to the transistor 21, which is mirrored by currents in respective transistors 13 and 15, as described above. In conventional driver circuits for controlling the write head of a disk drive, the steady state voltage levels to which the two terminals of write head 19 settle are both typically near either the high reference voltage level Vdd or the low reference voltage level Vss.
One problem encountered in disk drives employing existing drive circuitry for the write head coil 19 is that the wires or lines connecting the write head coil 19 to the write drive circuitry are located proximally to the wires or lines connecting the read head to the read channel circuitry (not shown in FIG. 1). The close proximity between the wires capacitively couples the wires together. As a result, voltage spikes or other voltage transitions appearing on the lines connected to write head coil 19 may have a greater tendency to appear as noise on the lines connected to the read head of the disk drive and potentially damage the read head as a result.
Because of the inductive nature of the write head coil 19 and because conventional steady state voltage levels for the write head terminals are approximately near the high reference voltage level Vcc, a relatively sizeable voltage spike typically may be generated on a terminal of write head 19 (the terminal of write head 19 having a voltage signal experiencing a falling transition) during the time that the current passing through write head 19 transitions from one direction to another. In other words, a relatively sizeable voltage spike appears on a terminal of the write head when the write head transitions between steady states. As can be seen, this relatively sizeable voltage spike may be capacitively coupled to the lines associated with the read head of the disk drive and thereby damage the read head.
As data rates increase, the rates at which the heads can accurately write the data to the magnetic media is limited by the speed at which the flux in the coil 19 (and its associated components) can be reversed. Relatedly, the amplitude of voltage spikes appearing on a write head terminal (and noise appearing on the corresponding read head terminals) is based in part upon the rate of flux reversal. The maximum data rate is thus limited to the maximum physical flux reversal rate of the write head drive circuitry and the maximum allowable noise that may be tolerated at the read head during the period of flux reversal.
What is needed, therefore, is a method and circuit for driving an inductive load of the type used in conjunction with a write head of a disk drive with a signal that enables a maximum flux reversal rate in the driver coil in an absence of an appreciable amount of noise on lines capacitively coupled to the inductive load.